1 [transitive,intransitive]to tell somebody about something, especially something dangerous or unpleasant that is likely to happen, so that they can avoid itwarn somebodyI tried to warn him, but he wouldn't listen.If you're thinking of getting a dog, be warned—they take a lot of time and money.warn (somebody) about/against somebody/somethingHe warned us against pickpockets.warn (somebody) of somethingPolice have warned of possible delays.warn (somebody) that…She was warned that if she did it again she would lose her job.warn somebody what, how, etc…I had been warned what to expect.warn (somebody) + speech‘Beware of pickpockets,’ she warned (him).Express YourselfWarning people of dangerYou may need to tell someone that they are in danger or advise them not to do something dangerous:Look out! There's a car coming.Be careful. It can be quite dangerous on that path.Watch out. That's not a very safe place at night.Make sure you keep hold of your bag.I wouldn't do that if I were you.

2 [intransitive,transitive]to strongly advise somebody to do or not to do something in order to avoid danger or punishmentsynonymadvisewarn (somebody) against/about somethingThe guidebook warns against walking alone at night.warn somebody (to do something)He warned Billy to keep away from his daughter.They were warned not to climb the mountain in such bad weather.‘I’m warning you!’ said James, losing his patience.

3[transitive]warn somebody (for something)(in sport, etc.)to give somebody an official warning after they have broken a ruleThe referee warned him for dangerous play.

Word OriginOld Englishwar(e)nian, wearnian, from a West Germanic base meaning ‘be cautious’.Extra examplesHaving been duly warned that I would get nowhere with my application, I went right ahead and applied anyway.I did try to warn you.I must warn you that some of these animals are extremely dangerous.I thought I should warn her about it.My mother constantly warned me not to go into teaching.No one had warned us about the unbearable heat.She claimed doctors had failed to warn her of the risks involved.The chancellor bluntly warned the Cabinet to axe public spending or face higher taxes.The report warns of the dangers of obesity.They warned us of the risks involved.We were specifically warned against buying the house.We were warned against drinking the local water.You will get better—but be warned, it may be a long process.‘I’m warning you!’ said James, losing his patience.I tried to warn him, but he wouldn’t listen.If you’re thinking of getting a dog, be warned —they take a lot of time and money.She warned Billy to keep away from her daughter.The commander had been warned of the attack.Phrasal Verbswarn somebody off (something)